This invention relates to novel azoic pigments, and more specifically to azoic pigments containing a fine powder of silica as a core and having a high color density and excellent fastness, solvent resistance, water resistance, heat resistance and light permeability, a process for production thereof, and to the use of these azoic pigments in a toner for electrophotography.
It was recently reported that Sudan I, an azoic dye, is formed on a nucleus of silica gel by esterifying silica gel with phenol or a phenylalkanol utilizing the reactivity of its surface silanol groups to introduce a phenyl group into the surface of silica gel, and thereafter subjecting the phenyl group to nitration, reduction, diazotization and coupling (see Shikizai (Color Materials), 55 [5]280-287, 1982).
The azoic pigment formed by using esterified silica gel, however, has various defects. For example, the esterification of silica gel requires high temperatures, and the yield of the ester is low. Moreover, the resulting ester linkage is highly polarized and is susceptible to hydrolysis.